Preheat oven to 350 degrees. Add cupcake liners to muffin pans.
In a large bowl, combine the cake mix, water, oil, and eggs.
Beat ingredients on low for one minute. Then increase speed to medium and beat for one minute more.
Stir hot cocoa mix into the cake batter.
Fill muffin cups 2/3 full with batter. Bake at 350 degrees for 18-20 minutes or until a toothpick inserted into the center of a cupcake comes out clean.
Remove cupcakes from oven and allow to cool completely.
In a large bowl, cream together butter and vanilla. Then, add the marshmallow fluff and beat on high until the frosting is creamy.
Add the frosting to a piping bag, attach a large star or round tip. Starting at the outer edge, pipe around the cupcake in a circle, slowly working in towards the center.
Add two mini Oreos to each cupcake to create Mickey's ears.
To complete the look, glue two yellow buttons to the cupcake liner.